Methods of representing and analysing images

ABSTRACT

A method of representing at least one image comprises deriving at least one descriptor based on color information and color interrelation information for at least one region of the image, the descriptor having at least one descriptor element, derived using values of pixels in said region, wherein at least one descriptor element for a region is derived using a non-wavelet transform. The representations may be used for image comparisons.

The invention relates to a method and apparatus for representing animage or group of images, and a method and apparatus for comparingimages, eg for detecting discontinuities in image sequences or for imageretrieval.

In Lienhart, R., “Comparison of Automatic Shot Boundary DetectionAlgorithms”, In Proceedings of Image and Video Processing VII 1999,Proc. SPIE 3656-29, pp. 290-301, January 1999, a method is presented forthe detection of shot transitions in a video sequence. For each frame inthe sequence, a 3-dimensional histogram in the RGB colour space iscreated. The difference between consecutive frames in the sequence isthen calculated as the difference between their respective histograms,calculated as the sum of absolute bin-wise differences. Shot transitionsare then identified by searching for the distances which are above apredetermined fixed threshold. Thus, this method detects shottransitions based solely on spatially insensitive colour contentinformation. Therefore, this method does not make use of the wealth ofinformation that is present in the spatial arrangement andinterrelations of colours.

In Zabih, R., Miller, J., Mai, K., “A Feature-Based Algorithm forDetecting and Classifying Scene Breaks”, In Proceedings of 1995 3^(rd)ACM International Conference on Multimedia, San Francisco, Calif. USA,pp. 189-200, 1995, a different method is presented for the detection ofshot transitions in a video sequence. For each frame in the sequence, anedge map is calculated. The difference between consecutive frames in thesequence is then calculated based on the number of edges which arepresent in the first frame but not in the second and the number of edgeswhich are present in the second frame but not in the first. Then, sharppeaks in the time series of this difference measure indicate thepresence of a shot transition. Thus, this method detects shottransitions based solely on edge information, which is one type ofspatial interrelation information. Although the rationale is correct,this method does not make use of the wealth of information that ispresent in the colour content of the frame. Furthermore, the edge mapcreation process is computationally expensive and is meant to revealonly the strongest colour discontinuities within the frame. Furthermore,the method is quite sensitive to motion. Thus, the authors suggest theuse of an image registration technique to counteract this shortcoming,but such processes are computationally expensive.

In Dailianas, A., Allen, R. B., England, P., “Comparison of AutomaticVideo Segmentation Algorithms”, SPIE Integration Issues in LargeCommercial Media Delivery Systems, vol. 2615, pp. 2-16, October 1995,another method is presented for the detection of shot transitions in avideo sequence. The difference between consecutive frames in thesequence is calculated as the sum of absolute pixel-wise differences.Shot transitions are then identified by searching for the distanceswhich are above a predetermined fixed threshold. Thus, this methoddetects shot transitions based solely on spatially sensitive colourcontent information. Although the rationale is correct, this method doesnot make use of the wealth of information that is present in the spatialinterrelations of colours. Furthermore, such a simple processing of thevideo results in high sensitivity to noise and motion. A motioncompensation algorithm could address the motion sensitivity problem, butsuch processes are computationally expensive.

In Xiong, W., “Shot Boundary Detection”, US 2003/0091235 A1, PublishedMay 15, 2003, Filed Nov. 9, 2001, a method is presented for thedetection of shot transitions based on the combination of differenttypes of information. That method comprises calculating a block-baseddifference between two frames and, if it exceeds a fixed threshold,declaring a candidate shot transition. In this case, the shot transitionis verified by requiring that colour and/or edge differences between thetwo frames also exceed fixed thresholds. For the calculation of theblock-based difference, the frames are divided into blocks and blockaverages are calculated. Then, the difference between correspondingblocks is thresholded to determine if two blocks are similar ordifferent, and the number of different blocks between two frames isthresholded to determine if two frames are similar or different. Thecolour difference is the sum of absolute bin-wise differences, while theedge difference uses edge histograms, capturing edge magnitude anddirection information.

In Nakajima, Y., Sugano, M., Yanagihara, H., for KDDI CORPORATION (JP),“Picture Searching Apparatus”, US 2004/0091044 A1, Published May 13,2004, Filed Aug. 15, 2003, Priority Sep. 20, 2002, a method is presentedfor the detection of shot transitions based on (a) correlation betweenimages, (b) correlation between subsampled images, (c) motion betweenimages and (d) motion between subsampled images. There, the correlationbetween images and between subsampled images is measured as a pixel-wisedifference or a histogram difference and the motion between images andbetween subsampled images is measured based on various motion vectordifferences.

In Jafarkhani, H., Shahraray, B., for AT&T CORP. (US), “Method forAnalyzing Video”, U.S. Pat. No. 6,542,619 B1, patent Apr. 1, 2003, FiledApr. 13, 2000, a shot transition detection method is presented whichcomprises creating two one dimensional projections of a video frame,i.e. row and column projections, performing a wavelet transform on eachprojection and retaining only the high frequency components (i.e. thewavelet coefficients), and auto-correlating the high frequencycomponents of each transform. For a series of video frames, a shottransition is indicated when the resultant auto-correlation coefficienttime curves exhibit a predetermined maximum value. Thus, that methodemploys spatially sensitive colour content and interrelationinformation, provided by the wavelet transform, but that informationrelates not to frames but to frame projections, resulting is greatinformation loss.

In Jacobs, C. E., Finkelstein, A., Salesin, D. H., “Fast MultiresolutionImage Querying”, In Proceedings of 1995 ACM SIGGRAPH Conference, LosAngeles Calif., USA, August 9-11, pp. 277-286, 1995, a method forretrieving images similar to a given image is presented. With thatmethod images are initially represented by their Haar waveletdecomposition. Then, this decomposition is truncated, i.e. only thescaling function coefficient (average intensity) and a very small numberof the largest magnitude Haar wavelet coefficients are retained. Then,the truncated decomposition is quantised, i.e. only the signs of thewavelet coefficients are retained. Thus, a single image descriptor isformed that characterises the image for the purposes of image retrieval.

In Zhuang, Z.-Y., Hsu, C.-T., Chen, H.-Y., Ouhyoung, M., Wu, J.-L.,“Efficient Multiresolution Scene Change detection by WaveletTransformation”, In Proceedings of 1997 IEEE International Conference onConsumer Electronics ICCE '97, Taipei, Taiwan, June 11-13, pp. 250-251,1997, a method for the detection of shot transitions is proposed thatproceeds to characterise video frames in the same manner described inJacobs, C. E., Finkelstein, A., Salesin, D. H., “Fast MultiresolutionImage Querying”, In Proceedings of 1995 ACM SIGGRAPH Conference, LosAngeles Calif., USA, August 9-11, pp. 277-286, 1995. The differencebetween the methods of Jacobs, C. E., Finkelstein, A., Salesin, D. H.,“Fast Multiresolution Image Querying”, In Proceedings of 1995 ACMSIGGRAPH Conference, Los Angeles Calif., USA, August 9-11, pp. 277-286,1995 and Zhuang, Z.-Y., Hsu, C.-T., Chen, H.-Y., Ouhyoung, M., Wu,J.-L., “Efficient Multiresolution Scene Change detection by WaveletTransformation”, In Proceedings of 1997 IEEE International Conference onConsumer Electronics ICCE '97, Taipei, Taiwan, June 11-13, pp. 250-251,1997 is that with the method of Zhuang, Z.-Y., Hsu, C.-T., Chen, H.-Y.,Ouhyoung, M., Wu, J.-L., “Efficient Multiresolution Scene Changedetection by Wavelet Transformation”, In Proceedings of 1997 IEEEInternational Conference on Consumer Electronics ICCE '97, Taipei,Taiwan, June 11-13, pp. 250-251, 1997 the perimeter of frames isdiscarded and frames are reduced to only their central parts. Such anapproach leads to great information loss and can result in false videosegmentation and/or great over-segmentation when significant motion ispresent in the video.

A deficiency that is common with both the methods of Jacobs, C. E.,Finkelstein, A., Salesin, D. H., “Fast Multiresolution Image Querying”,In Proceedings of 1995 ACM SIGGRAPH Conference, Los Angeles Calif., USA,August 9-11, pp. 277-286, 1995 and Zhuang, Z.-Y., Hsu, C.-T., Chen,H.-Y., Ouhyoung, M., Wu, J.-L., “Efficient Multiresolution Scene Changedetection by Wavelet Transformation”, In Proceedings of 1997 IEEEInternational Conference on Consumer Electronics ICCE '97, Taipei,Taiwan, June 11-13, pp. 250-251, 1997 is the assumption that the Haardecomposition can be efficiently truncated by retaining only a verysmall number of the largest magnitude coefficients. To put this incontext, a multi-scale Haar decomposition of an image plane starting at128×128 pixels down to 2×2 pixels produces 16383 coefficients. For thepurposes of image retrieval or video processing, such a feature vectormay indeed be too large for a fast and efficient implementation.However, truncating this vector to a very small number of coefficientson the basis of magnitude, e.g. the 40 or 60 coefficients with thelargest magnitude as the authors suggest, is a mechanical way orreducing its dimensionality and, as those skilled in the art know,results in descriptors which give a spatially non-uniform representationof the image, are extremely susceptible to noise, are also verysusceptible to high video motion and intra-shot lighting effects forvideo segmentation, to name a few problems. Quantising the truncatedseries by retaining only the signs amplifies the problem.

Another drawback with the methods of Jacobs, C. E., Finkelstein, A.,Salesin, D. H., “Fast Multiresolution Image Querying”, In Proceedings of1995 ACM SIGGRAPH Conference, Los Angeles Calif., USA, August 9-11, pp.277-286, 1995 and Zhuang, Z.-Y., Hsu, C.-T., Chen, H.-Y., Ouhyoung, M.,Wu, J.-L., “Efficient Multiresolution Scene Change detection by WaveletTransformation”, In Proceedings of 1997 IEEE International Conference onConsumer Electronics ICCE '97, Taipei, Taiwan, June 11-13, pp. 250-251,1997 is that the calculation of the Haar decomposition entails anincreased amount of computations, due to the fact that the coefficientsfor an image region are computed based on all the pixel values withinthat region.

Here, we propose methods for assessing the similarity between images,for example for the retrieval of images from a set of images that aresimilar to a given image or for the detection of frame discontinuities,such as shot transitions or lighting and other effects, in digitalvideo. The methods rely on the extraction of image descriptors capturingspatially sensitive colour content and interrelation information at oneor more image scales and across one or more image channels. Onecharacteristic of the methods is that, unlike the previous methods, thedescriptors may be extracted so that they capture spatially sensitivecolour content and interrelation information uniformly across the imageplanes and at multiple scales, but have a reduced size compared to theaforementioned Haar decomposition, thus reducing the need for anartificial truncation process. Another characteristic of the methods isthat, unlike the previous methods, the descriptors may be extracted sothat they allow perfect reconstruction of the original image, but have areduced computational load compared to the aforementioned Haardecomposition.

In view of the above, an object of the invention is to provide animproved method for assessing the similarity between images, for examplefor the retrieval of images from a set of images that are similar to agiven image or for the detection of frame discontinuities, such as shottransitions or lighting and other effects, in digital video.

For video segmentation, the proposed methodology allows therepresentation of both uncompressed and MPEG compressed digital videoframes. For the latter, I frames can be represented withoutdecompression.

Aspects of the invention are set out in the accompanying claims.

Features of an embodiment include:

(i) extraction of image descriptors capturing spatially sensitive colourcontent and interrelation information at one or more image scales andacross one or more image channels, the descriptor of at least onescale-specific channel-specific image representation characterised by

all the regional descriptor elements of at least one descriptor elementtype being calculated using only a subset of the pixels of therespective regions.

and/or

the pixels used in the calculation of all the regional descriptorelements of all descriptor element types being only a subset of thepixels of the respective regions.

and/or

the total number of descriptor elements being less than the number ofpixels of said representation, said property being inherent in thedescriptor extraction process without reliance on any post-processing ofthe descriptors, for example truncation;

(ii) combining said descriptors in a single multi-scale multi-channeldescriptor; and

(iii) use of the resultant descriptors in assessing the similaritybetween images.

Specific embodiments of the invention are outlined below, with referenceto the accompanying drawings, of which:

FIG. 1 illustrates the block averaging resampling process for the Ychannel according to an embodiment of the invention;

FIGS. 2 A-D illustrate the calculation of the descriptor for a scale inone embodiment of the invention;

FIGS. 3 A-D illustrate the calculation of the descriptor for a scaleaccording to a further embodiment of the invention;

FIG. 4 is a flow diagram for comparing a plurality of images accordingto an embodiment of the invention;

FIG. 5 is a schematic illustration of a processing device suitable forthe implementation of embodiments of the invention.

A first embodiment of the invention relates to descriptor extraction.

Consider a digital image F_(i)(x,y) where (x,y) represent spatialcoordinates, with x=0 . . . M−1 and y=0 . . . N−1. In one embodiment ofthe invention, F_(i) is represented in the colour space YC_(b)C_(r),although this is not restrictive and the invention is applicable to anycolour space of any number of channels. In one embodiment of theinvention, the spatial resolution of F_(i) is 720×576 pixels, i.e. M=720and N=576, although this is not restrictive and the invention isapplicable to any spatial resolution. In one embodiment of theinvention, the highest resolution at which the descriptor is to beextracted is chosen to be 64×64 pixels, although this is not restrictiveand the invention is applicable to other resolutions. Thus, F_(i) isresampled to give the 64×64 pixel image F_(i(64×64)). In one embodimentof the invention, this resampling process is a simple block averagingprocess as shown in FIG. 1, although the choice of the resamplingmechanism is not restrictive. More specifically, FIG. 1 illustrates theblock averaging resampling process for the Y channel, although this isnot restrictive and the process can be used for any channel of anycolour space. In another embodiment of the invention, the highest scaleat which a descriptor is to be extracted coincides with the resolutionof F_(i) and no resampling is initially required.

FIG. 2 illustrates the calculation of the descriptor for this scale inone embodiment of the invention. More specifically, FIG. 2 illustratesthe calculation of the descriptor for the Y channel, although this isnot restrictive and a similar approach is applied to all colourchannels. In FIG. 2A a 2×2 window is applied to the top left corner ofthe image. This window specifies a local neighbourhood. For the pixelsin this neighbourhood F^(Y) _(i(64×64))(0,0), F^(Y) _(i(64×64))(1,0),F^(Y) _(i(64×64))(0,1) and F^(Y) _(i(64×64))(1,1) the descriptorelements χ₁ to χ₄ can be calculated as:χ₁=(F ^(Y) _(i(64×64))(0,0)+F ^(Y) _(i(64×64))(1,0)+F ^(Y)_(i(64×64))(0,1)+F ^(Y) _(i(64×64))(1,1))/4  (1)χ₂=(F ^(Y) _(i(64×64))(0,0)−F ^(Y) _(i(64×64))(1,0))/2  (2)χ₃=(F ^(Y) _(i(64×64))(1,0)−F ^(Y) _(i(64×64))(1,1))/2  (3)χ₄=(F ^(Y) _(i(64×64))(1,1)−F ^(Y) _(i(64×64))(0,1))/2  (4)

In the equations above, the element χ₁ captures colour contentinformation, while the elements χ₂, χ₃ and χ₄ capture colourinterrelation information.

Denoting V^(Y) _(i(64×64)) the descriptor of F^(Y) _(i(64×64)), thesevalues are written into the descriptor asV ^(Y) _(i(64×64))(1)=χ₁ , V ^(Y) _(i(64×64))(2)=χ₂ , V ^(Y)_(i(64×64))(3)=χ₃ , V ^(Y) _(i(64×64))(4)=χ₄  (5)

Next, as shown in FIG. 2B, the 2×2 window slides two pixels to the rightto form a new neighbourhood comprising pixels F^(Y) _(i(64×64))(2,0),F^(Y) _(i(64×64))(3,0), F^(Y) _(i(64×64))(2,1) and F^(Y)_(i(64×64))(3,1). The descriptor elements χ₁ to χ₄ are again calculatedfor the new pixel values and then stored in the descriptor positionsV^(Y) _(i(64×64))(5) to V^(Y) _(i(64×64))(8). As shown in FIGS. 2C and2D once all the calculations for a set of rows are completed, thesliding window moves two pixels down and again starts sliding from leftto right, until the entire image has been processed. This completes thecalculation of the descriptor V^(Y) _(i(64×64)) which captures theimage's spatially sensitive colour content and interrelations, for the Yplane at a scale of 64×64 pixels.

The invention outlined here comprises calculating descriptors thatcapture spatially sensitive colour content and interrelationinformation.

Another embodiment replaces descriptor elements χ₁ to χ₄ with descriptorelements ψ₁ and ψ₂ derived as set out below:ψ₁=(F ^(Y) _(i(64×64))(0,0)+F ^(Y) _(i(64×64))(1,0)+F ^(Y)_(i(64×64))(0,1)+F ^(Y) _(i(64×64))(1,1))/4  (1)ψ₂=max(F ^(Y) _(i(64×64))(0,0), F ^(Y) _(i(64×64))(1,0), F ^(Y)_((64×64))(0,1), F ^(Y) _(i(64×64))(1,1))−min(F ^(Y) _(i(64×64))(0,0), F^(Y) _(i(64×64))(1,0), F ^(Y) _(i(64×64))(0,1), F ^(Y)_(i(64×64))(1,1))  (II)

In the equations (I) and (II) shown above, ψ₁ captures colour contentinformation while ψ₂ captures colour interrelation information. Thenumber of descriptor elements is less than the number of pixels used toderive the descriptor elements.

Yet another embodiment replaces descriptor elements χ₁ to χ₄ withdescriptor elements φ₁-φ₄ derived as set out below:φ₁=(F ^(Y) _(i(64×64))(0,0)+F ^(Y) _(i(64×64))(1,0)+F ^(Y)_(i(64×64))(0,1)+F ^(Y) _(i(64×64))(1,1))/4  (III)φ₂=(F ^(Y) _(i(64×64))(0,0)−F ^(Y) _(i(64×64))(1,0))/2  (IV)φ₃=(F ^(Y) _(i(64×64))(0,0)−F ^(Y) _(i(64×64))(1,1))/2  (V)φ₄=(F ^(Y) _(i(64×64))(0,0)−F ^(Y) _(i(64×64))(0,1))/2  (VI)

In the equations (III)-(VI) shown above, φ₁ captures colour contentinformation while φ₂-φ₄ captures colour interrelation information.

In another embodiment of the invention descriptor elements are extractedaccording to the scheme of FIG. 3. In FIG. 3A a 4×4 window is applied tothe top left corner of the image. This window specifies a localneighbourhood comprising 16 pixels. For the pixels in this neighbourhoodthe following descriptor elements are calculated

$\begin{matrix}{{\psi_{1} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)/16}}\;} & (6) \\{{\psi_{2} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)/16}}\;} & (7) \\{{\psi_{3} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)/16}}\;} & (8) \\{{\psi_{4} = {{\max\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)} - {\min\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)}}}\;} & (9)\end{matrix}$

In the equations above, ψ₁ captures colour content information and ψ₂,ψ₃ and ψ₄ capture colour interrelation information. Those skilled in artwill understand that the equations (6)-(9) may be modified to reduce thenumber of required calculations.

These values may be written into the descriptor V^(Y) _(i(64×64)) alongthe lines of equation (5) and then, as shown in FIG. 3B, the 4×4 windowslides four pixels to the right to form a new neighbourhood, descriptorelements are again calculated for the new pixel values, and so on. Asshown in FIGS. 3C and 3D once all the calculations for a set of rows arecompleted, the sliding window moves two pixels down and again startssliding from left to right, until the entire image has been processed.

In alternate embodiments of the invention, the calculation of colourcontent and/or interrelation descriptor elements may rely on functionsof the neighbourhood pixels such as the median, the maximum, theminimum, etc.

Furthermore, although the previous descriptions consider slidingneighbourhood windows of size 2×2 and 4×4 pixels, this is notrestrictive. Alternative embodiments can specify any size and/or ageometry and/or a sliding step for the neighbourhood window, resultingin overlapping or non-overlapping windows, for example to suit givencomputational needs or the neighbourhood measurement process itself.

In the previous descriptions, the same neighbourhood window anddescriptor extraction process are used at all locations of F^(Y)_(i(64×64)). In alternative implementations this need not be so.Alternative embodiments may employ different neighbourhood parametersand/or descriptor extraction processes at different locations in theimage. For example, an alternative embodiment of the invention uses afast descriptor extraction process at the perimeter of the image and aslower but more powerful descriptor extraction process at the centre ofthe image.

Next, F^(Y) _(i(64×64)) is resampled to a new lower scale. In oneembodiment, this new scale is 32×32 pixels. Then, the new descriptorV^(Y) _(i(32×32)) is calculated in exactly the same manner as for the64×64 image. Note that it may be the case that no computations areactually required for this resampling. As can be seen in equation (1),the new pixel values were calculated earlier as χ₁ values and are storedin V^(Y) _(i(64×64))(1), V^(Y) _(i(64×64))(5), V^(Y) _(i(64×64))(9), . .. , V^(Y) _(i(64×64))(4093). Thus, in one embodiment, the descriptorsV^(Y) _(i(64×64)), V^(Y) _(i(32×32)), V^(Y) _(i(16×16)), V^(Y)_(i(8×8)), V^(Y) _(i(4×4)), and V^(Y) _(i(2×2)) are calculated.

The invention outlined here comprises extracting descriptors at one ormore scales of the image F_(i). In the previous descriptions, the finestor maximum or highest scale, the coarsest or minimum or lowest scale,the number of scales and the relation between scales, are representativeof only one possible embodiment of the invention and alternativeembodiments may employ different such parameters in order, for example,to match the computational capabilities of a system.

In one embodiment of the invention, certain elements are removed fromthese scale-specific descriptors before subsequent processing. In oneembodiment of the invention, for the descriptor extraction processoutlined in equations (1)-(4) at scales 2^(b)×2^(b), 2^(b−1)×2^(b−1),2^(b−2)×2^(b−2) and so on, e.g. 64×64, 32×32, 16×16, and so on, all theχ₁ elements in V^(Y) _(i) may be removed except for the χ₁ elements atthe lowest scale. This is because the χ₁ elements at scale2^(b−k)×2^(b−k) can be directly calculated from the χ₁ values at scale2^(b−k−1)×2^(b−k−1) and the χ₂, χ₃, and χ₄ elements at scale2^(b−k)×2^(b−k). Note that this property will not always hold. Forexample, it will not apply if the descriptors are calculated at scales2^(b)×2^(b), 2^(b−2)×2^(b−2), 2^(b−4)×2^(b−4) and so on, e.g. 64×64,16×16, 4×4, and so on. Also, it will not hold if only a subset of theχ₁, χ₂, χ₃ and χ₄ elements are calculated as part of the descriptorextraction process. The same property applies to the transform ofequations (III)-(VI).

Applying the same descriptor extraction process to the C_(b) and C_(r)channels of F_(i) gives the descriptors V^(Cb) _(i(64×64)), . . . ,V^(Cb) _(i(2×2)) and V^(Cr) _(i(64×64)), . . . , V^(Cr) _(i(2×2)). Allthese scale-specific channel-specific descriptors capture the image'sspatially sensitive colour content and interrelation information atmultiple scales and across multiple channels.

In the previous descriptions, F_(i) is assumed to be in the YC_(b)C_(r)colour space and all the channels are used in the extraction of thedescriptors. Alternative implementations may represent the image in anew colour space before the calculation of the descriptors and/or useonly a subset of the colour planes in the calculation of itsdescriptors.

In the previous descriptions, the descriptors across the differentcolour channels of F_(i) are calculated at the same scales. Inalternative implementations this need not be so. For example, analternative embodiment of the invention calculates the Y channeldescriptors at more scales and/or different scales than the C_(b) and/orC_(r) channel descriptors. Such fine tuning results in a betteradaptation to a specific image or video material and can improve theperformance.

In the previous descriptions, the same neighbourhood window anddescriptor extraction process is used for all the colour channels ofF_(i). In alternative implementations this need not be so. Alternativeembodiments may employ different neighbourhood parameters and/ordescriptor extraction processes for each channel. For example, analternative embodiment of the invention uses a fast descriptorextraction process for the C_(b) and C_(r) channels and a slower butmore powerful descriptor extraction process for the Y channel. Such finetuning results in a better adaptation to a specific image or videomaterial and can improve the performance of a system.

In one embodiment of the invention, some or all of the descriptorelements are represented as real numbers with an integer and afractional part. In another embodiment of the invention some or all ofthe descriptor elements are represented in integer form, for example bytruncating or by rounding their fractional part. In yet anotherembodiment of the invention some or all of the descriptor elements arerepresented in integer form by scaling. For example, equations (1)-(4)produce real numbers where the fractional part can take only threevalues, namely 0.25, 0.50 and 0.75. Multiplying the descriptor elementχ₁ with the value 4 and the descriptor elements χ₂, χ₃ and χ₄ with thevalue 2 will result in integer representations without any informationloss. In yet another embodiment of the invention some or all of thedescriptor elements are represented in integer form by any means and arethen quantised to shorter values, for example 3 or 4 bits long.

Thus, in one embodiment of the invention, image F_(i) is represented byits descriptors V^(C1) _(i(C1SCq)), . . . , V^(C1) _(i(C1SC1)), V^(C2)_(i(C2SCq)), . . . , V^(C2) _(i(C2SC1)), . . . , V^(Cp) _(i(CpSCq)), . .. , V^(Cp) _(i(CpSC1)) where C1, C2, . . . , Cp are a first, second andp^(th) colour channel of F_(i), C1SCq is the highest descriptor scalefor channel C1, C1SC1 is the lowest descriptor scale for channel C1,C2SCq is the highest descriptor scale for channel C2, C2SC1 is thelowest descriptor scale for channel C2, and so on. All thesescale-specific channel-specific descriptors capture the image'sspatially sensitive colour content and interrelation information atmultiple scales (preferably more than one) and across multiple channels(preferably more than one).

Such descriptors are then fused, i.e. concatenated, into a singledescriptor for subsequent processing. This is illustrated in FIG. 4.

In that embodiment, in step 4100, scale-specific channel-specificdescriptors are extracted as previously explained. In step 4250, thesedescriptors are fused into a multi-scale multi-channel descriptor V_(i).In step 4400, the descriptors V_(i) and V_(j) are used for thecalculation of a similarity measure D_(j,i) between images F_(i) andF_(j). In step 4900, the similarity measure is used in the assessment ofthe similarity or dissimilarity of the two images.

The various steps of this process will now be examined more closely.

In step 4400, there are many options for the calculation of a similaritymeasure. For example, in one embodiment of the invention the L1 distancebetween two descriptors V_(i) and V_(j), of n elements each, may becalculated as the similarity measure, given by

$\begin{matrix}{{L\; 1_{j,i}} = {\sum\limits_{l = 0}^{n}{{{V_{j}(l)} - {V_{i}(l)}}}}} & (10)\end{matrix}$

The invention outlined here comprises using the image descriptors forthe calculation of a similarity measure, comprising at least two values,between images.

The L1 distance of equation (10) for the calculation of descriptordistances relates to only one possible implementation of the invention.An alternative implementation replaces equation (10) with equation (11)below, which calculates the L2 distance

$\begin{matrix}{{L\; 2_{j,i}} = \sqrt{\sum\limits_{l = 0}^{n}\left( {{{V_{j}(l)} - {V_{i}(l)}}} \right)^{2}}} & (11)\end{matrix}$

There is great variety of such distance metrics available in therelevant literature.

Furthermore, an alternative implementation replaces equation (10) withequation (12) below, which calculates the weighted L1 distance

$\begin{matrix}{{L\; 1_{j,i}} = {\sum\limits_{l = 0}^{n}\left( {{w(l)} \cdot {{{V_{j}(l)} - {V_{i}(l)}}}} \right)}} & (12)\end{matrix}$

where w(l) is the weighting factor for the l^(th) descriptor elements.Furthermore, the application of weights may also be used in conjunctionwith the L2 distance of equation (11) or any other suitable distancemetric available in the literature.

There are many options for the processing of the similarity measure. Inone embodiment of the invention, in step 4900 a thresholding function isapplied to the similarity measure D_(j,i). That is, for a thresholdvalue thr, ifD_(j,i)≧thr  (13)

the two images are deemed dissimilar and a decision “dissimilar” is madein step 4900. Otherwise, the two images are deemed similar and adecision “similar” is produced.

The embodiment outlined here comprises using said similarity measure,comprising at least two values, to ascertain the similarity ordissimilarity between images.

The thresholding scheme outlined above results in step 4900 producing abinary decision, for example “1” denoting “similar” and “0” denoting“dissimilar”. In another embodiment of the invention, the decision ofstep 4900 is not binary but comprises at least three values. In such acase, the term “likelihood measure” is better than the term “decision”to describe the result of processing step 4900. For example, in oneembodiment of the invention, step 4900 uses three thresholds. If asimilarity measure is below a first threshold, a similarity likelihoodmeasure of “0”, denoting “dissimilar” is produced. If a similaritymeasure is above the first threshold but below a second threshold, whichis higher than the first threshold, a similarity likelihood measure of“1”, denoting “probably dissimilar” is produced. If a similarity measureis above the second threshold but below a third threshold, which ishigher than the second threshold, a similarity likelihood measure of“2”, denoting “probably similar” is produced. If a similarity measure isabove the third threshold, a similarity likelihood measure of “3”,denoting “similar” is produced. In yet another embodiment of theinvention, the likelihood measure is produced not through thresholdingbut by normalisation of the similarity measure to the fixed range, e.g.between 0 and 1.

In a preferred embodiment of the invention according to FIG. 4, step4900 produces a binary decision or a multi-valued likelihood measure,where said likelihood measure might be the similarity measure producedin step 4400 after normalisation to a fixed range, e.g. between 0 and 1.In an alternative embodiment of the invention, step 4900 is notimplemented at all, and the final result of the processing according tothe invention is the unnormalised similarity measure produced by step4400.

The semantics of the similarity assessment that is made in step 4900depend on the application that the invention is implemented for.

In one embodiment of the invention, F_(i) and F_(j) are temporallyordered images. For example, they are frames of a video F_(z), and theaim is to find whether a frame discontinuity exists between F_(j) andF_(i), such as a shot transition or lighting effect and the such. Then,the decision at step 4900 will be whether the similarity measure betweenF_(j) and F_(i) indicates such a frame discontinuity. Such a systemwould be used for the purpose of segmenting a video. In such anapplication, the preferred output of step 4900 would be a binarydecision, allowing the segmentation of the video.

In another embodiment of the invention F_(j) and F_(i) are individualimages with no temporal ordering. For example, F_(j) is an image of aset of images F_(z) and F_(i) is another image, either from inside oroutside F_(z), and the aim is to find those images in F_(z) that aresimilar to F_(i). Then, the decision of step 4900 will be whether thesimilarity measure between F_(j) and F_(i) indicates that the images aresimilar. Such a system would be used for image retrieval by query or forthe categorisation of images in a database. In such an application, thepreferred output of step 4900 would be a multi-valued likelihoodmeasure, preferable the similarity measure normalised to a fixed range,instead of a binary decision, allowing the creation of a ranked list ofsimilar images.

The above are but two possible applications of the invention.

In one embodiment of the invention, one or both of the descriptors V_(i)and V_(j) of step 4400 represent not images but groups of images.

For example, in one embodiment of the invention V_(j) is a function,such as the average or the median, of the descriptors V_(j1), V_(j2), .. . , V_(jg) of images F_(j1), F_(j2), . . . , F_(jg) which form thecluster of images F_(j). For example, F_(j) could be a cluster of imagesrepresenting cars. For an image F_(i), ascertaining the similaritybetween descriptors V_(i) and V_(j) will then reveal not whether F_(i)is similar to a particular image, but whether it is similar to aparticular class or cluster of images. A set of images F_(z) couldcontain any number of such clusters, and such a processing would revealwhich particular cluster a given image matches best. Furthermore, acluster may be represented not only by V_(j), but also by otherstatistics such as the covariance matrix of the descriptors of itsimages, which allows the calculation of more complex similarity measuresin step 4400, such as the Mahalanobis distance or the maximum likelihoodmeasure.

In another embodiment of the invention for the segmentation of video,V_(j) is a function, such as the average or the median, of thedescriptors V_(j1), V_(j2), . . . , V_(jg) of frames F_(j1), F_(j2), . .. , F_(jg). These frames might all precede the frame F_(i), or theymight all follow it, or some may precede it and some may follow it.

The embodiment outlined here comprises using said similarity measure,comprising at least two values, to ascertain the similarity ordissimilarity between images. Thus,

In the previous embodiments of the invention for video segmentation, instep 4900, the similarity measure between two frames F_(i) and F_(j) isused for the detection of frame discontinuities. An alternativeembodiment of the invention uses not only this similarity measure butalso the similarity measures between frames around frames F_(i) andF_(j).

For example, in one embodiment of the invention j=i−1 and the detectionof frame discontinuities relies on the similarity measuresD_(i+k−1,i+k)∀kε[−t₁,t₂]. In a preferred embodiment, t1=t2=t. Thus, theframes F_(i+k) form a temporal window centred on frame F_(i). Then, aframe discontinuity is detected between F_(i−1) and F_(i) if and only if

$\begin{matrix}{D_{{i - 1},i} \geq \begin{matrix}D_{{i + k - 1},{i + k}} & {{\forall{k \in \left\lbrack {{- t},t} \right\rbrack}},{k \neq {0\mspace{14mu}{and}}}}\end{matrix}} & (14) \\{{\frac{D_{{i - 1},i}}{D_{{i + p - 1},{i + p}}} \geq {{thr}\mspace{14mu}{with}\mspace{14mu} D_{{i + p - 1},{i + p}}} \geq {D_{{i + k - 1},{i + k}}\mspace{14mu}{\forall k}}},{p \in \left\lbrack {{- t},t} \right\rbrack},k,{p \neq 0}} & (15)\end{matrix}$

According to equations (14) and (15), each similarity measure D_(i−1,i)is considered inside a temporal window of size 2t+1, centred onD_(i−1,i). Then, a discontinuity is detected between frames F_(i−1) andF_(i) if and only if D_(i−1,i) is the largest distance within thetemporal window and at least thr times larger than the second largestdistance inside the temporal window. Such a temporally adaptive schemeaims in reducing the number of false detections resulting from highmotion levels between frames. There are several such schemes ofprocessing such a time series for the detection of framediscontinuities.

It should be clearly understood that in a preferred embodiment of theinvention, descriptors are extracted at many scales and across multiplechannels. However, in alternative embodiments of the invention this neednot be so. For example, in one embodiment of the invention, descriptorsare extracted at multiple scales but for only a single channel. Inanother embodiment of the invention, descriptors are extracted for asingle scale, but across multiple channels. In yet another embodiment ofthe invention, descriptors are extracted at a single scale and for asingle channel.

Furthermore, in the previous embodiments the processing of all pairs ofconsecutive frames was considered for the detection of framediscontinuities. Alternative embodiments may process frames moresparsely. An alternative embodiment processes only every 4^(th) frame,while yet another embodiment processes only every 16^(th) frame. Suchenhancements further increase the speed of a video segmentation system.

Furthermore, the above descriptions relate to the processing ofuncompressed digital video, where each frame is represented by its pixelvalues. The invention can also be applied directly to MPEG compressedvideo without decompression.

The nature of MPEG video is well known to those skilled in the art andis expertly described in the book “The MPEG Handbook” by John Watkinson,“The MPEG Handbook: MPEG-1, MPEG-2, MPEG-4”, Focal Press, Oxford 2001,ISBN:0-240-51656-7.

In one embodiment of the invention for MPEG compressed video, only the Iframes of the video are processed. More specifically, only the DCrepresentations of the I frames are processed, i.e. for each block of8×8 pixels, only the DC component of its DCT transform is taken. Ineffect, the DC representation of an I frame is simply a resampledversion of the original frame before its compression using 8×8 blockaveraging. Since DC components can be retrieved directly from thecompressed video stream without decompression, it is evident that theinvention is directly applicable for the processing of the I frames ofMPEG compressed video.

The invention can be implemented for example in a computer system, withsuitable software and/or hardware modifications. For example, theinvention can be implemented using a computer or similar device havingcontrol or processing means such as a processor or control device, datastorage means, including image storage means, such as memory, magneticstorage, CD, DVD etc, data output means such as a display or monitor orprinter, data input means such as a keyboard, and image input means suchas a scanner, or any combination of such components together withadditional components. Aspects of the invention can be provided insoftware and/or hardware form, or in an application-specific apparatusor application-specific modules can be provided, such as integratedcircuits. Components of a system in an apparatus according to anembodiment of the invention may be provided remotely from othercomponents, for example, over the internet.

A block diagram of a suitable apparatus in the form of a computer systemis shown in FIG. 5 where a control unit 10100 is connected to an imagestorage means 10200, a descriptor storage means 10300 and a display10400. The image storage device 10200 stores data corresponding to theimages to be represented or compared according to the aforementionedembodiments. This may be data corresponding to a single image, aplurality of images, clusters of images, sequences of images or anentire image database. The descriptor storage means 10300, which may bea database, stores all derived descriptors including sub-descriptorscalculated by the control unit 10100. Results of the operation of thecontrol unit are displayed to a user on display 10400.

The terms “scale” and “resolution” are used interchangeably herein. Aregion of an image can mean the whole image or part of an image.

1. A method of representing an image comprising using a processor toperform the steps of: deriving at least one descriptor based on colourinformation and colour interrelation information for at least one regionof the image, the descriptor being derived using values of pixels insaid region and having at least two descriptor elements that arefunctions of values of pixels in said region, wherein at least one ofsaid descriptor elements for said region is derived using only a subsetof pixels in said region, wherein deriving said at least one descriptorincludes performing the steps of: identifying said region of the image;determining the values of pixels in said region; performing a firstcalculation on the pixel values of said region to determine a firstvalue that represents colour information of said region; performing asecond different calculation on the pixel values of said region todetermine a second value that represents colour interrelationinformation of said region, wherein at least one of said first andsecond calculations involves examining only a subset of pixels in theregion; and forming a descriptor containing at least said first andsecond values as said at least two descriptor elements.
 2. The method ofclaim 1 wherein each of a plurality of descriptors elements for a regionare derived using only a subset of pixels in said region.
 3. The methodof claim 1 or claim 2 wherein all the descriptor elements for a regionare derived using only a subset of pixels in said region.
 4. The methodof claim 1 wherein the calculations used to derive the descriptorelements are such that the resulting number of descriptor elements isless than the number of pixels used to derive the descriptor elements.5. The method of claim 1, wherein at least one descriptor elementcaptures colour information.
 6. The method of claim 1, wherein at leastone descriptor element captures colour interrelation information.
 7. Themethod of claim 1, wherein at least one descriptor element represents apixel average for said region, or at least one descriptor elementrepresents the difference function involving pixels in said region, orat least one descriptor element represents a pixel value range in saidregion.
 8. The method of claim 1 wherein at least one descriptor elementrepresents a difference between two pixel values, or a differencebetween two sums of pixel values.
 9. The method of claim 7 wherein thedescriptor elements are derived using equations of the form of thefollowing equations 1 to 4: $\begin{matrix}{\chi_{1} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)}} \right)/4}} & (1) \\{\chi_{2} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)}} \right)/2}} & (2) \\{\chi_{3} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)}} \right)/2}} & (3) \\{\chi_{4} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)}} \right)/2}} & (4)\end{matrix}$ for any scale of said image.
 10. The method of claim 7wherein the descriptor elements are derived using equations of the formof the following equations I and II: $\begin{matrix}{\psi_{1} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)}} \right)/4}} & (I) \\{{\psi_{2} = {{\max\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)}} \right)} - {{\quad\quad}{\min\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)}} \right)}}}},} & ({II})\end{matrix}$ or the following equations III to VI: $\begin{matrix}{\phi_{1} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)}} \right)/4}} & ({III}) \\{\phi_{2} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)}} \right)/2}} & ({IV}) \\{\phi_{3} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)}} \right)/2}} & (V) \\{{\phi_{4} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)}} \right)/2}},} & ({VI})\end{matrix}$ or the following equations 6 to 9: $\begin{matrix}{\psi_{1} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)/16}} & (6) \\{\psi_{2} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)/16}} & (7) \\{\psi_{3} = {\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)} - {F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)} + {F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)/16}} & (8) \\{\psi_{4} = {{\max\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)} - {\min\left( {{F_{i{({64 \times 64})}}^{Y}\left( {0,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,0} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,1} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,2} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {0,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {1,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {2,3} \right)},{F_{i{({64 \times 64})}}^{Y}\left( {3,3} \right)}} \right)}}} & (9)\end{matrix}$ for any scale of said image.
 11. The method of claim 1wherein descriptors are produced for a plurality of over-lapping ornon-overlapping regions of an image.
 12. The method of claim 1comprising processing the image to produce a plurality ofrepresentations of the image at a plurality of scales; and derivingdescriptors for said plurality of scales, or processing the image foreach of a plurality of colour channels and deriving descriptors for eachof said channels.
 13. The method of claim 12 wherein the same scales areused for different colour channels.
 14. The method of claim 12 whereindifferent scales are used for different channels.
 15. The method ofclaim 1 wherein different processes for deriving descriptors are usedfor one or more of : different regions, different scales, and differentcolour channels.
 16. The method of claim 1 wherein the descriptors arederived directly from MPEG encoded I frames without decoding.
 17. Themethod of claim 1 comprising associating a plurality of descriptors foran image to form a single descriptor.
 18. A method of representing agroup of images, comprising deriving a representation of each imageusing the method of claim 1, and using said representations to representsaid group of images.
 19. The method of claim 18 comprising deriving afunction of said representations to represent the group of images. 20.The method of claim 19 wherein said function comprises a statisticalmeasure.
 21. A representation of an image or group of images derivedusing a method of claim
 1. 22. A method of comparing images or groups ofimages comprising comparing representations according to claim
 21. 23.The method of claim 22 comprising comparing images which are part of asequence of images and wherein the result of the comparison is used todetermine whether a frame discontinuity or event has occurred betweenthe images.
 24. The method of claim 22 used for image retrieval.
 25. Amethod of encoding an image or group of images using the method ofclaim
 1. 26. A method of decoding an image or sequence of images encodedusing the method of claim
 25. 27. Use of data encoded according to themethod of claim 25, by means of transmission, relay or reception.
 28. Anapparatus for representing an image or group of images according to themethod of claim
 1. 29. Apparatus for comparing images or groups ofimages according to the method of claim
 1. 30. The apparatus of claim 28or claim 29 comprising storage means for storing image data or imagedescriptors, and processing means for executing said method.
 31. Acomputer-readable storage medium encoded with a computer program forexecuting the method of claim 1, 22, 25, or
 26. 32. The method of claim1, wherein the at least one descriptor element comprises a plurality ofdescriptor elements including a colour information element and a colourinterrelation information element, wherein the colour interrelationinformation element is derived using a function of pixels values of apixel neighbourhood in said region, the function representing aninterrelationship between pixel values in said neighbourhood.